1. Field of the Invention
The present invention relates to pressure sensors, more particularly, to air gauge devices used in lithography systems.
2. Background Art
Conventional style air gauges are used to measure the location of a wafer surface in a number of lithography tools. These conventional air gauges include a bridge having a measurement nozzle located near the wafer's surface. The conventional air gauges typically also include a separate reference nozzle located near, or in the same environment as, measurement components. As a gap between the wafer and the measurement nozzle changes, the flow rate through the measurement nozzle is altered, and a change in differential pressure or flow in the bridge is detected.
In general, although the measurement nozzle may be retractable, its position is fixed during the measurement process. Likewise, the gap between a reference nozzle and its target may be adjustable, but remains fixed during the measurement process. The gap measurements made by these conventional air gauges are most accurate when the wafer surface is at the nominal gap where the flow through the bridge is nearly balanced, and becomes less accurate as the measurement gap moves away from the nominal value. Off null, the air gauge becomes sensitive to changes and ambient pressure, and the relationship between gap and sensed differential flow or pressure is non-linear.
The air gauge can be used at typical standoffs of less than approximately 0.150 millimeters (mm). At the physical scales of interest to wafer surface sensing, a substantial increase in an air gauge standoff value (H) is not possible, as the measurement sensitivity drops quite drastically, approximately to H−3.3. At such small standoffs, there is a possibility of a collision between the air gauge nozzle and, for example, a wafer surface. Also, to the extent that the air gauge is required to accurately measure a range of wafer positions, its accuracy is limited.
What is needed, therefore, is a method and system for facilitating measurements where the air gauge will always be operated at a favorable standoff, maximizing its performance and useful measurement range. More specifically, what is needed is a gauging device that will minimize the risk of a collision between the air gauge nozzle and the surface of the wafer.